For example, in a manufacturing process of a semiconductor device, as a thermal processing, there is an oxidation step wherein an oxide film is formed on a surface of a semiconductor wafer, which is an object to be processed. As a method for the oxidation treatment, there is a method to bring a semiconductor wafer into contact with water vapor at a predetermined process temperature and thus to oxidize the same (wet oxidization). In order to carry out the method, there is known an oxidation processing unit (a thermal processing unit) wherein a burning unit that causes hydrogen gas and oxygen gas to react (burn) to generate water vapor is independently arranged outside a processing furnace, and wherein the water vapor generated by the burning unit is supplied into the processing furnace to carry out a heat treatment, as shown in JP Laid-Open Publication No. 63-210501 or the like.
In addition, as a thermal processing unit, there are a normal-pressure type of unit having a normal-pressure gas-discharging system and another type of unit, by which a treatment can be carried out under a reduced pressure, having a normal-pressure gas-discharging system and a reduced-pressure gas-discharging system.
In a conventional normal-pressure type of thermal processing unit, a discharging-pressure-controlling valve, which is a butterfly-valve type or another type of adjusting its own opening degree via a stepping motor and a spring, and a differential-pressure type of pressure sensor are provided in a normal-pressure gas-discharging system that discharges inside gas from a processing furnace at a predetermined discharging-pressure, in order to control the discharging-pressure.
On the other hand, in a conventional reduced-pressure type of thermal processing unit, by which a treatment can be carried out under a reduced pressure, a gas-discharging system from a processing furnace branches into a normal-pressure gas-discharging system and a reduced-pressure gas-discharging system. A switching valve is provided at the branching portion, and the discharging-pressure-controlling valve and the pressure sensor, which are described above, are provided in the normal-pressure gas-discharging system in order to control the discharging-pressure. In addition, a combination valve and a pressure sensor are provided in the reduced-pressure gas-discharging system in order to carry out a reduced-pressure control.
However, in both the normal-pressure type of thermal processing unit and the reduced-pressure type of thermal processing unit, if the discharging-pressure-controlling valve is a butterfly-valve type, the water vapor may condense to generate a water screen between the valve and a pipe, which may make the control unstable. Thus, in order to avoid the above disadvantage, it was necessary to provide atmospheric-air-introducing ports on the front side and the rear side of the valve. In addition, if the discharging-pressure-controlling valve is another type of valve which adjusts its own opening degree via a stepping motor and a spring, in order to make the motion of the valve smooth and hence to make the control performance stable, it was necessary to introduce an inert gas such as N2 gas toward the valve. Thus, running-costs for the inert gas were necessary. In addition, in the reduced-pressure type of thermal processing unit, the switching valve is necessary, which leads to structural complication.